The first one, OnTop, allow you to Sculpt, Mask or Polypaint on the top of a surface, especially a flat surface. It’s a little bit different from the Automasking >> Cavity Masking. This brush is using the brush Depth Mask with various settings. I strongly advise you to use a large brush and trying to remain on the top of the surfaces to avoid artifacts on the side of the object. A tip is to create masks, inverse the mask, blur and sharpen it twice.

This is a small video of the OnTop in action, used to create a mask.

The second one, Onborder, is a brush which work on border of small extrusions and is based on Automasking >> CavityMask.